In some combustion engine vehicles, an operator may perform a diagnostic procedure for detecting the drive train power output. For example, in a vehicle with a torque converter transmission, a stall test may be performed to estimate the power output of an internal combustion engine. The stall test may include placing the transmission in gear, commanding full engine fueling while resisting vehicle motion with a brake, and observing the resultant engine speed. A resulting engine speed lower than a design specification may be indicative of low engine power. In an electric vehicle, a method is needed for diagnosing the power output performance of a traction motor.